The Vikings had no real access to the personal lives of various Asgardians nor can we assume their historical documents are accurate.

This.

Quote:

Originally Posted by Incredible Hans

So you mean all the legends (or most of them) aren't true? Interesting idea.

But in the real world, all people familiar with Norse mythology know that Loki is of frost giant heritage. Do all people on earth in the MCU know that? And if they do, why is Loki's heritage common knowledge on earth but a secret in Asgard (only Odin and Frigga knew it before Loki found out).

So Kenneth Branagh created some problems with his version of Thor. There are, however, several explanations how you could solve this problems:

A) All that "myth stuff" isn't true. These stories were made up by the Vikings. Thor never lifted the Midgard Serpent, he never disguised as Freya, Loki never killed Balder. And in the MCU, there is no mythological Viking story that tells of Loki's giant heritage. So the fact that Loki is a frost giant was a secret in Asgard and Midgard.

B) All the "myth stuff" is true. However, it didn't happen to the current versions of Thor, Loki, Odin etc. Ragnarok already took place once, Asgard had been destroyed, all the gods were reborn. Thor in his first life lifted the Midgard serpent (and that's what the Vikings tell us about in their sagas). The current Thor didn't, he was a small boy during the Viking age. The first Loki was a son of a frost giant (so that's the reason why the Vikings and all scholars of Norse mythology knew it), but the current Loki didn't know that this was also true for him.

C) All the "myth stuff" is true, but it didn't take place during the Viking era, but some centuries later. Loki and Thor weren grown-ups then. In the MCU, there are no Viking stories about adult Thor and Loki - all these stories come from the Icelander Snorri Sturleson (who was a Christian and no Norse pagean) who lived in the 12th/13th century. So Snorri knew Thor and Loki when they were adult and wrote down their stories.

and possibly this in bold too.... Not completely but to an extent. There would have to be some fiction in the mythology, since it was from spoken tales eventually written down as stories by ancient people, but it may have been based on some real events/characters (real as in, in the MCU) from a time prior to the flashbacks in Thor 1. A previous Ragnarok. The Asgardians either may not be aware what the Midgardians wrote about them in their books or don't consider it truth since they have no recollection of those things happening (except possibly I could see Odin being aware of the past)